reference electrodes
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Reference Electrodes
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Reference ElectrodesBasic function of a reference electrode is to
maintain a constant electrical potential against which deviations may be measured.
The desirable characteristics that a reference electrode should passes are
1. It should be easy to construct and
2. It should develop potentials which are reproducible even if small currents are passed.
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Reference ElectrodesA reference is an electrode that has the half-cell potential known,
constant, and completely insensitive to the composition of the solution under study. In conjunction with this reference is the indicator or working electrode, whose response depends upon the analyte concentration.
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Reference Electrodes
Ideal Reference Electrode: Is reversible and obeys the Nernst equation Exhibits a potential that is constant with time Returns to its original potential after being
subjected to small currents Exhibits little hysteresis with temperature
cycling
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Reference Electrodes
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It’s a primary reference electrode. Its potential is considered to be zero.
Electrode reaction: half cell: pt, H2 / H+ (1N) Eo = zero d-Limitation1. It is difficult to be used and
to keep H2 gas at one atmosphere during all determinations.
2. It needs periodical replating of Pt. Sheet with Pt. Black
Standard Hydrogen Electrode
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Reference Electrodes
Calomel Electrodes:
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Reference Electrodes
Silver/Silver Chloride Electrodes:
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Ag/AgClDisadvantage of silver-silver chloride
electrode1. It is more difficult to prepare than SCE.2. AgCI in the electrode has large solubility in
saturated KCl
Advantage of Ag-AgCI electrodes over SCE.1. It has better thermal stability.2. Less toxicity and environmental problems with
consequent cleanup and disposal difficulties.
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Indicator electrode Ecell=Eindicator-Ereference
It must be: (a) give a rapid response and(b) its response must be reproducible. Metallic electrodes: where the redox reaction
takes place at the electrode surface.Membrane (specific or ion selective)
electrodes: where charge exchange takes place at a specific surfaces and as a result a potential is developed.
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Metallic Indicator Electrodes
There are four types of metallic indicator electrodes:
1. Electrodes of the first kind.
2. Electrodes of the second kind.
3. Electrodes of the third kind.
4. Redox electrodes.
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Metallic Indicator Electrodes
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Membrane Indicator Electrodes Properties: Minimal solubility. A necessary property of an ion-selective
medium is that its solubility in analyte solutions approaches zero.
Electrical conductivity. A membrane must exhibit some electrical conductivity. Generally, this conduction takes the form of migration of singly charged ions within the membrane.
Selective reactivity with the analyte. A membrane or some species contained within the membrane matrix must be capable of selectively binding the analyte ion. Three types of binding:
Ion-exchangeCrystallizationComplexation
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Membrane Indicator Electrodes
Glass Electrodes:
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Membrane Indicator Electrodes
Glass Electrodes: PotentialThe boundary potential.The potential of the internal Ag/AgCl
reference electrode.A small asymmetry potential.
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Membrane Indicator Electrodes
Liquid Membrane Electrodes:Cation exchangersAnion exchangersNeutral macrocyclic compounds, which
selectively complex certain cations
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Membrane Indicator Electrodes
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Ion-Selective Field-Effect Transistors (ISFETs)
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Molecular-Selective Electrode Systems Gas-Sensing Probes: Microporous materials- manufactured from
hydrophobic polymers that have a porosity of about 70% and a pore size of less than 1m, and are about 0.1mm thick.
Homogeneous films- solid polymeric substances through which the analyte gas passes by dissolving in the membrane, diffusing, and then desolvating into the internal solution.
They are usually thinner than microporous in order to hasten the transfer of gas and thus the rate of response of the system.
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Molecular-Selective Electrode Systems
Gas-Sensing Probes:
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Instruments for Measuring Cell Potentials Direct-Reading InstrumentsCommercial Instruments
Utility General-purpose Expanded-scale Research